1. Field of the Invention
The present disclosure relates to an image forming apparatus which forms an image by an electrostatic latent image.
2. Description of the Related Art
In an image forming apparatus such as color laser beam printers, digital copiers, and the like, the image is temporarily formed on an intermediate transfer member. The image formed on the intermediate transfer member is then transferred onto a recording medium (for example, sheet). Then, the image is printed. As the intermediate transfer member, for example, a belt which is formed into an endless shape is used. Transferring the image formed on the intermediate transfer member to the recording medium is called a secondary transfer.
FIG. 16 is a diagram for explaining a secondary transfer mechanism. A driven roller 110 and a secondary transfer roller 112 are arranged to face each other. When a driving roller 108 rotates at a constant speed, the intermediate transfer belt 107 moves at a constant speed. A sheet S is conveyed along with a guide 131 by a conveyance roller 130 and the secondary roller 112, which rotate at a constant speed. Thereafter, the secondary transfer is performed to the sheet S. In this case, the intermediate transfer belt 107 is designed to move at the same speed at which the sheet S is conveyed. Actually, however, this is not always the case. In the following, a particular example is explained using FIGS. 17A and 17B.
FIG. 17A is a diagram for explaining an arrangement of the driven roller 110 and the secondary transfer roller 112 in a case where the intermediate transfer belt 107 moves at the same speed at which the sheet S is conveyed. A longitudinal direction of the column-shaped driven roller 110 is defined as a main scanning direction. A direction which is vertical to the main scanning direction is defined as a sub-scanning direction. In the arrangement shown in FIG. 17A, a rotary shaft 110z of the driven roller 110 is in parallel with a rotary shaft 112z of the secondary transfer roller 112. Therefore, a pressure (nip pressure) of a portion at which the driving roller 110 and the secondary transfer roller 112 contact each other (nip portion) is constant regardless of a position in the main scanning direction (main scanning position). Thereby, the sheet S is conveyed at the same speed at which the intermediate transfer belt 107 moves.
FIG. 17B is a diagram for explaining an arrangement of the driven roller 110 and the secondary transfer roller 112 in a case where the intermediate transfer belt 107 does not move at the same speed at which the sheet S is conveyed. In the arrangement shown in FIG. 17B, a rotary shaft 110z of the driven roller 110 is not in parallel with a rotary shaft 112z of the secondary transfer roller 112. Due to this, the nip pressure of the driven roller 110 and the secondary transfer roller 112 differ according to the main scanning position. For example, a distance between the shafts of the driven roller 110 and the secondary transfer roller 112 at the main scanning position (x) in FIG. 17B is longer than that in the arrangement shown in FIG. 17A. Therefore, the nip pressure at the main scanning position (x), located at a left side in FIG. 17B, is relatively low. On the other hand, a distance between the shafts of the driven roller 110 and the secondary transfer roller 112 at the main scanning position (y) in the arrangement shown in FIG. 15B is shorter than that in the arrangement shown in FIG. 15A. Therefore, the nip pressure at the main scanning direction (y) is relatively high. As mentioned, if the nip pressure differs according to the main scanning position, the pressure applied to the sheet S by the driven roller 110 and the secondary transfer roller 112 also differs according to the main scanning direction. Further, the higher the nip pressure is, the faster the sheet S is conveyed. Also, the lower the nip pressure is, the slower the sheet S is conveyed. This is because as the nip pressure is high, the frictional force to the sheet S also enhances, which enables easy transmission of the rotational force to the sheet S. Due to this, the conveyance speed of the sheet S differs according to the main scanning position in the arrangement shown in FIG. 17B.
FIG. 17B shows a case where a print operation is performed in a state where the nip pressure is different for every main scanning position. In this case, as the sheet S passes through the nip portion, it performs a sector-like rotation. As a result, a deviation is caused in the secondary transfer. For example, when printing an image shown in FIG. 18A, as shown in FIG. 18B, the shape is printed in a distorted shape. The distortion is compared to a sector. A position at low nip pressure at the main scanning position is an outer peripheral side of the sector. Also, a position at high nip pressure at the main scanning position is an inner peripheral side of the sector. In the following, such distortion is called a sector deformation.
FIG. 18B shows a sheet having experienced the sector deformation, in which distortion amount b1 and distortion amount b2 at the main scanning positions respectively are about 0.1[mm]. Further, the distortion amount b3 at a position of a sub-scanning direction (sub-scanning position) is about 0.5 [mm]. As above, the distortion amount b3 at the sub-scanning position is several times larger than the distortion amounts b1 and b2 at the main scanning positions. Due to this, the distortion of the sub-scanning direction by the sector deformation significantly affects the print image quality, which is a problem.
To this problem, an image forming apparatus as disclosed in US Patent Application Publication No. US2007/0139715(A1) intends to realize correction processing to the sector deformation by image data conversion processing. In particular, by detecting an output image formed on a printed sheet, a deformation parameter of the sector deformation is obtained. Based on the result as obtained, in the following printing operation, image data is converted in advance to cancel the occurrence of the distortion caused by the sector deformation. This is to avoid the occurrence of any defective image caused by the sector deformation.
On the other hand, in the image forming apparatus as disclosed in US Patent Application Publication No. US2007/0139715(A1), a sector deformation parameter is obtained by detecting an entire image of the output image formed on the printed sheet. Therefore, the image forming apparatus is required to have many sensors for detection, a configuration for highly recognizing the detected image, and large capacity memory. As a result, the time required to manufacture the image forming apparatus and parts for constituting the image forming apparatus largely increase. Thereby, a manufacturing cost of the image forming apparatus largely increases, which is a problem.
It is a main object of the present disclosure to provide an image forming apparatus which is capable of easily setting a parameter for avoiding an occurrence of defective image by the sector deformation.